Wet granular materials, such as those produced by sedimentation, filtration, or centrifuging from slurries, generally require some dewatering, partial or complete, in order to improve their handling and storage qualities. For example, the underflow from thickeners used to collect the water employed in coal cleaning processes contains from 25 percent to 35 percent coal and 65 percent to 75 percent water. The use of vacuum filters can reduce the amount of water to the range of 25 percent to an excess of 30 percent depending upon the relative coarseness of the solid particles. Centrifuges, which are costlier than vacuum filters in terms of capital, energy and maintenance expenses, may be utilized to reduce the moisture content to about 18 percent. However, the filter cakes produced by either centrifuges or vacuum filters are sloppy and subject to freezing in cold weather. Additionally, these filter cakes usually contain more moisture than is desirable, even after mixing with drier, coarser fractions, for different end uses such as pulverized coal combustion or as feed to coke-making ovens.
In order to facilitate handling and storage and to reduce the moisture content of the total washed material to an acceptable level of approximately 6 percent of final use, thermal drying is practiced. Thermal drying is very energy intensive and also necessitates the extensive use of pollution control means. In the thermal drying of moist coal, for example, a fire hazard may exist during the drying process due to overdrying caused by the variability of the moisture content of the coal being dried.
Another means of dewatering materials involves the use of presses. Typically, presses are utilized with fibrous materials which, when compressed, produce a matted mass that prevents the loss of solids and the occlusion of any drain openings. Press dewatering of granular materials, while being less costly than any of the above referenced methods, also has a variety of attendant difficulties. The major problem is that the drains or openings in the devices used in such a process tend to become plugged with the material being compressed. This plugging inhibits the escape of the expressed fluid from the materials. Various means have been developed to circumvent this problem including the use of self-cleaning tapered openings or the mechanical or fluid cleaning of the openings. Examples of such prior art are disclosed in U.S. Pat. Nos. 1,448,774; 2,398,135; 3,520,411; 4,043,832 and 4,159,947.
In U.S. Pat. No. 4,208,188 a dewatering apparatus is disclosed which utilizes a porous drainage member having a quasi-triangular porous structure similar to a woven screen having a mesh size in the range of about 50 to 100 microns. During compression, water drains through the drainage screen and the solid particles consolidate and tend to agglomerate and bridge the interstices of the screen rather than following the tortuous path through the screen. Use of the above described means leads to either increased construction costs or increased complexity in the structure of the dewatering apparatus over presses employing the self-cleaning slotted openings of the present invention. While this patent does not address the problem of the plugging of the porous drainage member during operation, it appears that this member would be subject to this problem in contrast to an apparatus employing the invention disclosed herein.